Prior to the development of PMG's, aircraft typically used wound field generating systems as a power supply. With weight and size considerations becoming of increased importance, however, PMG devices of the type shown in U.S. Pat. Nos. 4,305,031 and 4,654,577 were developed. These generators were considered relatively small and lightweight devices for generating electrical power from a motive power source because they do not require a separate electrical power source for field excitation.
A PMG does not require a complex cooling apparatus and eliminates the need for a converter by driving the system with a constant speed drive to limit the maximum voltage excursion in the case of a system fault. T h e output voltage of a PMG is, however, a function of the relative speed between the rotor and stator and of the current drawn by a load. This drawback has prevented PMG's from being used as a main power source in aircraft in lieu of wound field machines having exciters and rotating rectifiers driven at high speed. These high speeds required complex cooling apparatus to dissipate heat developed on the windings and in the rotating rectifiers.
When used in a generator, there is a set voltage on the generator which varies as the load varies and which cannot be controlled electronically. It has been found that the rotors in PMGs create a voltage control problem as the load varies. A dual permanent magnet generator concept controls voltage by shifting two high speed rotors in and out of phase to keep the voltage constant as the load varies. For example, at a 2 per unit (P.U.) load, the magnetic poles are lined up, whereas at no load, the magnetic poles would be 78.degree. out of phase as previously noted. If the main high speed rotors of the PMG which, at no load, are out of phase with each other by about 78.degree. were kept in the same phase relationship as the generator load is increased, the output voltage would droop.
An alternating current generator having a twin permanent magnet rotor which is adjustable in response to output voltage is known from U.S. Pat. No. 3,713,015. As disclosed therein, only one rotor was actuated using a lever to force sliding motion for achieving a relative rotation of the rotors. This known arrangement involved considerable friction and therefor required significant power for overcoming inertia to angularly adjust the rotors.
Another PMG system utilized a plurality of movable stator winding disks disposed in a plurality of air gaps intermediate the magnetic structure. Stator windings were disposed on the winding disks with the respective winding phases connected to one another in series. This system required many bearings to permit movement of the disks relative to the housing and a complex interconnection scheme for the stator windings.
Another PMG system used two radial flux magnetic structures on a rotor for producing first and second magnetic fields with one of the structures movable relative to the other by means of an actuator. Stator windings on the first and second magnetic fields developed output voltage as a result of relative rotation. The output voltage magnitude was controlled by varying the placement of one field relative to another field. This system required the actuator to be on the rotor, and this requirement rendered the actuator complex, large and expensive. A phase-controlled rectifier bridge was used but was relatively complex due to the necessity of having substantial controls to accurately fire the controlled rectifiers in the bridge at appropriate points in the output power waveform to develop DC output power. A relatively large filter had to be connected to the output of the bridge to provide smoothing, and this filter had to have means for dissipating heat developed in the filter components
U.S. Pat. No. 4,663,581 shows such a voltage regulated PMG system in which outputs of two or more radial flux PMG's are combined and adjusted relative to each other to regulate output voltage. This system was intended to avoid drawbacks in the aforementioned PMG systems by requiring relatively few bearings, avoiding complex interconnection between the stator windings and the need for an actuator on the rotor, and providing a high power density machine of smaller size and weight for a given output power rating which can be driven at high speeds without substantial cooling. This system required a hydraulic positioning actuator to adjust the position of one stator relative to another in accordance with the magnitude of a control signal from a control circuit.
U.S. Pat. No. 4,728,841 also discloses a dual PMG system in which two relatively movable permanent magnet rotor assemblies are disposed in side-by-side relation to regulate voltage in response to generator loading changes or to compensate for changes in the rate of rotor rotation. This PMG incorporated a differential with first and second ring gears but utilized a more complex arrangement of the rotor body assemblies which can use rotor bodies of different mass or equal mass but which require control units of relatively complex construction, such as an hydraulic actuator with a double-acting piston of the type shown in U.S. Pat. No. 4,654,577. Such actuators add undesirably to the size, weight and complexity of the PMG system and detract from its power and reliability.
A PMG system using lefthand and righthand helical ball splines to effect adjustment of rotors relative to each other is disclosed in copending U.S. application Ser. No. 208,114, filed Jun. 17, 1988, now U.S. Pat. No. 4,879,484. This system comprises a stator having an output winding for producing an output voltage, a rotor within the stator, the rotor including first and second magnet rotors rotatable about a common axis and a mechanism for angular adjusting the first and second rotors relative to one another about the common axis. The angular adjustment of the rotors provides for rotation of each of the first and second rotors in opposite direction relative to the other for causing a relative angular adjustment of the rotors. Rotating each rotor simultaneously in opposite directions by half the required angle requires much less power to overcome inertia than rotating only one rotor the full angle. This simultaneous rotation of each of the rotors a like amount in opposite directions during a relative angular adjustment is accomplished using a common drive shaft extending along the common axis of the rotors for rotatably driving both the first and second rotors about the common axis. Left-and righthand helical ball splines are located between the driven shaft and the first and second rotors, respectively. Rotation of the first and second rotors in opposite directions relative to one another is accomplished by causing axial movement of the drive shaft along the common axis relative to the rotors. In that particular system, bearings are provided for rotatably supporting the rotors at essentially fixed locations along the common axis, and the drive shaft is permitted to move axially along the common axis relative to the rotors for angular adjustment of the rotors. However, the means for axially moving the drive shaft comprises a hydraulic drive means or an electric motor. In either case, the use of the oppositely directed helical ball splines together with the axial movement of the single drive shaft, reduces the friction required for angular adjustment of the rotors and thereby reduces the power necessary for accomplishing the adjustment.
A substantial problem with a PMG used, for instance in aircraft installations where weight is critical, has been until up to now how to actuate these high speed rotors in and out of phase while utilizing the least amount of power and weight. It was found necessary to use external power to actuate the rotors as described in detail above.